Research



CPK drawings of generations zero to three of our amide-based, acid-terminated dendrimers that were first prepared by Dr. James K. Young during his graduate studies. Construction was accomplished by repetitive DCC-promoted, amine and acid coupling of an aminotris(tert-butyl ester) monomer to give a new layer (generation) followed by peripheral ester conversion to an acid. Thus, starting with the tetraacid core, ideal terminal group progression increases from 4 to 12 to 36 to 108. Generations 4 and 5 (not shown) ideally possess 324 and 972 termini, respectively. Of course, due to the divergent method of preparation, defects (the failure of a monomer to react) become more pronounced at higher generations. However, it has been shown that 'dendritic character' is still maintained.





CPK drawings of generations 1 through 3 tert-butyl ester precursors of the acids pictured above. While there appears to be little open space in the G3 ester, there is room for permeation of guest molecules into the framework as determined by solvent accessible surface calculations and experiment.



The table below gives some pertinent data for this series of dendrimers.



Generation
Expected Number of Termini
Expected Formula Weight
0
4-Acid
424
1
12-Ester
2,015
1
12-Acid
1,341
2
36-Ester
6,112
2
36-Acid
4,092
3
108-Ester
18,404
3
108-Acid
12,345
4
324-Ester
55,280
4
324-Acid
37,102
5
972-Ester
165,909
5
972-Acid
111,373



The hydrodynamic radius of this series, from generations 1 — 5, as determined by diffusion data obtained by DOSY NMR experiments. The radius change with pH is due to expansion and contraction of the arms in response to deprotonation and protonation of the terminal carboxylic acid groups. Notably, at neutral pH the polyacid dendrimers are slightly larger suggesting that pH is not the sole factor in size determination. Other factors probably include hydrogen-bonding, ionic strength, counter ion composition, and temperature.



Generation
# of Termini
Radius (Acidic pH)
Radius (Neutral pH)
Radius (Basic pH)
1
12
8.24 Å
12.3 Å
11.8 Å
2
36
11.4 Å
17.3 Å
15.8 Å
3
108
17.3 Å
23.9 Å
21.9 Å
4
324
22.6 Å
33.1 Å
28.9 Å
5
972
25.4 Å
37.1 Å
32.3 Å



More Research: Self-assembly; Arborols.



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